1. Field of Technology
The present invention relates to technology for a voltage-controlled oscillator that is used in wireless communication devices having a portable terminal, and to a PLL circuit that uses the voltage-controlled oscillator.
2. Description of Related Art
A voltage-controlled oscillator (VCO) is used in portable wireless devices such as cell phones for frequency conversion operations converting transmission signals to high frequency signals for transmission and converting reception signals to low frequency signals for demodulation. These applications require a wide oscillation frequency range, the ability to freely adjust the oscillation frequency, and a high carrier-to-noise (C/N) ratio at the oscillation frequency.
Semiconductor devices used in the communications industry today often have an internal voltage-controlled oscillator. Spiral inductors are generally used when the inductor is also built in to the IC device. A wide oscillation frequency band is achieved in the built-in voltage-controlled oscillator by switching between spiral inductors.
An example of this type of conventional voltage-controlled oscillator is the oscillation circuit and inductance load difference circuit shown in FIG. 20 and taught in Japanese Unexamined Patent Appl. Pub. 2004-266718 corresponding to United States Patent Appl. Pub. US 2004/0183606 A1.
The oscillation circuit shown in FIG. 20 is composed of a differential inductance-capacitance resonance circuit and positive feedback circuit where the resonance circuit comprises capacitor C1 and an inductance load difference circuit comprising variable inductance units Lvar1 and Lvar2, and the positive feedback circuit comprises n-channel MOS transistors M1 and M2. The variable inductance units Lvar1 and Lvar2 each have first and second input/output (I/O) terminals with the second I/O terminals connected to a common external power supply node Vdd. The first I/O terminals are connected to output nodes OUT and OUTB, respectively. The capacitor C1 is also connected to the first I/O terminals of the variable inductance units Lvar1 and Lvar2. The oscillation frequency of the voltage-controlled oscillator can be determined from the inductance of the variable inductance unit and the capacitance.
The variable inductance units Lvar1 and Lvar2 vary the inductance and control the oscillation frequency by switching a plurality of switch circuits SW1, SW2, SW3, SW1d, SW2d, and SW3d disposed between a plurality of selected positions on the spiral wiring layer and the I/O terminals. The variable inductance units Lvar1 and Lvar2 form an inductor pair when switch SWndd of switch circuits SW1, SW2, SW3 connected between the first I/O terminals is ON at the same time as switch circuits SWn and SWnd.
See Japanese Unexamined Patent Appl. Pub. 2004-266718 corresponding to United States Patent Appl. Pub. US 2004/0183606 A1.
The variable inductance units taught in the patents cited above are composed of serial-parallel circuits comprising a plurality of inductors and a plurality of switch circuits, and changes the overall inductance in steps by turning the switch circuits on or off. As a result, the oscillation frequency of the voltage-controlled oscillator also changes in steps.
This arrangement enables increasing the bandwidth of the voltage-controlled oscillator to some degree but does not afford sufficiently fine-tuning the oscillation frequency because correcting for variation in the inductors built in to the IC device is deficient. The oscillation frequency band can also not be freely set, and correcting for the capacitance-voltage nonlinearity and temperature characteristic of a varactor diode is not possible.
The nonlinearity of the varactor diode also makes directly modulating the voltage-controlled oscillator difficult.